Paper sheets are produced from a continuous strip whose longitudinal edges are trimmed and that is then transversely cut into a succession of the sheets that are delivered one at a time one immediately after the other to a stacking apparatus of the type described in commonly owned U.S. Pat. No. 5,628,505 of Voss, U.S. Pat. No. 5,672,045 of Schmid, and U.S. Pat. No. 5,681,038 of Kollann. Such a stacking apparatus typically has a vertically displaceable platform on which is supported a pallet. The sheets are delivered by a conveyor to the stacking apparatus to form a stack on the pallet, which is lowered as the stack height increases so that each sheet has only to drop a minimal distance before coming to rest on top of the stack.
Each sheet is gripped upstream of the station between a lower belt or a group of parallel lower belts and a group of upper belts, but of course the lower belt ends at the input side of the station so that the sheets can drop downward. The upper belts normally therefore extend through the station and serve significantly to guide and position the incoming sheets.
In order to ensure clean separation of the sheets from the upper belts it is standard to provide one or more blow tubes. Such systems are described in East German patent 111,870 of Pretzsch, German patent 2,755,160 of Bodewein (U.S. equivalent 4,221,377), German patent document 3,323,052 of Henkenhaf, German patent 4,012,943 of Vits, Swiss patent 637,090 of Bodewein, and European 0,056,924 of Fitzparick. The blow tubes generally are simple tubes extending in the transport direction between the upper belts and formed with downwardly open holes from which respective jets of air are directed at the upper face of the sheets to push them down off the upper belts onto the stack.
As a rule these blow tubes are fairly expensive items to manufacture. They must be carefully shaped and the nozzle holes must be carefully formed. If nozzle fittings are provided to focus the jets, they must be constructed such that the leading edge of the incoming sheet does not catch on them. As a result blow tubes are a significant investment. Furthermore they are difficult to change or replace in the event of a format change in the sheets requiring a shorter or longer tube, so that one is often inclined to use overlong tubes that result in waste of air pressure and premature separation of the sheets from the upper conveyor belts.